Many scientific instruments require fluid switching valves for functions such as solvent selection, fraction collection, fluid redirection, stream sampling, or sample injection. These valves must have small diameter passages, typically in the range of 0.005 to 0.1 inch, that are well aligned and without cavities (xe2x80x9cdead volumesxe2x80x9d), so as to minimize dispersion as samples and fluid elements pass through them. A common architecture is shown in FIG. 1, where the stator 100A has six peripheral passages and one central passage, and the rotor 102A has one radial passage, thereby allowing selection of one of six solvents for direction out the central port, depending on the rotational position of the rotor. Sometimes there are many more peripheral ports. Another common architecture is shown in FIG. 2, where the rotor 102 contains alternating peripheral grooves 108. There are many other switching patterns.
In addition to low dispersion, other desirable features for valves used as scientific instruments include high inertness, low friction, and long lifetime.
This last characteristic, long lifetime, which is typically measured in numbers of actuations, has become increasingly important during the last ten years. The cycle time of scientific analysis has shortened in order to become more productive. As a consequence, valves with a short lifetime require frequent maintenance to replace one or more of the sealing parts. Formerly such maintenance might be required every six months, but now it may be required every week when using conventional valves with high duty cycles. The down time caused by such maintenance is undesirable, as it becomes a significant expense and slows productivity.
Lifetime can be defined as the number of actuations, or in the case of two-position valves, the number of cycles between position A-to-B-to-A, before the sealing parts need to be replaced due to excessive leakage. The amount of leakage that can be tolerated varies with the application. In one common use, high performance liquid chromatography using conventional columns at flow rates of 1 milliliter per minute, a leakage rate of 0.3 microliter per minute is commonly tolerated, but not one that is much larger, such as 3 microliters per minute.
Leakage can be from one or all of the ports or grooves radially outward to the extra-valve environment, i.e. to ambient, or leakage can be between ports which is called cross-port leakage. The latter is common and often is the more detrimental to function. For example, cross-port leakage in a valve used in the auto sampler of a liquid chromatograph can cause poor analytical precision due to errors in sample metering.
It should be understood that the type of valve under discussion does not have a two-state characteristic, leaking or not leaking. Rather, the amount of leakage is high when the fluid pressure is high, and is low when the fluid pressure is low, asymptotically approaching zero as the fluid pressure approaches zero. It is for this reason that the lifetime is described in terms of the leak rate exceeding a predetermined amount such as 0.3 microliter per minute at a predetermined fluid pressure such as 5000 psi.
The lifetime of the sealing is typically determined by adjusting the valve to hold a specified set pressure, while leaking no more than a specified rate, then cycling the valve and periodically testing its pressure holding capability. When this pressure holding capability drops a specified amount, the maximum lifetime is said to have been reached. For example, using a test apparatus, a valve is adjusted until it leaks 0.3 microliters per minute when pressurized to 5000 psi. It is then attached to an automated apparatus capable of repetitively actuating the valve. It is cycled 5,000 times, then returned to the test apparatus, where the pressure at which the leakage is no more than 0.3 microliters per minute is determined. The valve is again attached to the actuating apparatus and cycled another 5,000 times, after which it is again tested. This sequence is repeated until the pressure at which leakage is no more than 0.3 microliters per minute has dropped a specified amount. This specified amount might be, for example, 500 psi, which is 10% of the original set pressure. If the pressure required to keep the leak rate below 0.3 microliters per minute dropped to 4,500 psi after 20,000 cycles, and below 4,500 psi after 25,000 cycles, this valve would be said to have a lifetime of 20,000 cycles.
It is common to use a stator of metal such as stainless steel, so tubing connections can be attached in threaded holes, and to use a rotor of fluorocarbon-containing plastic for low friction sliding against the metal under a clamping force that presses the surfaces together at slightly more than the pressure of the fluid. Cross-port leakage is thought to be caused by scratches or depressions in the surface of the stator and/or rotor that form leak grooves. Such leak grooves provide a path for fluid flow when there is a pressure gradient between the ports. Lifetime is increased by delaying the onset, reducing the number, and minimizing the size of such leak grooves. In valves that are the subject of this invention, the design of surfaces to maximize lifetime is difficult to do from first principles. This is because, as is commonly understood, the subject of wear of plastic parts is of considerable complexity. An understanding of wear, and its related tribological (study of friction and wear) topics of friction and lubrication, involves topics such as asperity deformation, adhesion, modes of energy dissipation, molecular relaxation times, etc., each in itself a complex subject.
The limitations of the science of tribophysics cause the invention of longlife valves of the type being discussed to be driven by experimentation using a large variety of materials and surface treatments that would not necessarily be expected to produce good results. Indeed, in the experience of the inventors, little is predictable in the art of making valves.
For example, ceramic is an extremely wear resistant material that has been used as a counterface against polymeric rotors. However, the polymers that exhibit long lifetime against ceramic must be determined experimentally. Furthermore, when certain polymers are used as rotors and run against polished ceramic, the presence in the ceramic of relatively large pits does not necessarily cause excessive wear and short lifetime. Conversely, some extremely smooth ceramic surfaces cause high wear.
There are ceramic-polymer seal combinations that have long lifetimes. However, making a stator of ceramic is costly, primarily due to the size and complex shape, including the threaded ports. As an alternative, a ceramic stator face assembly can be placed on a metal stator, to provide long lifetime, as in the model 7750E-020 valve made by Rheodyne, L. P. The stator face is a ceramic plate that is sealed by a static seal to the face of a metal body, with the ceramic plate and metal body having aligned passages. The cost of such a stator face is less than the cost of a totally ceramic stator. However, this additional part adds volume to the valve passages, which increases dispersion. In some applications the added dispersion cannot be tolerated. It also results in an additional (static) sealing interface between the stator face and the bulk stator (metal body), which sometime requires that additional axial force be used on the stack of parts, in order to assure a fluid-tight static seal.
If the rotor is made of ceramic the cost is less than a ceramic stator, but the stator must then be the polymer part, which is problematic because it is too weak a material to use with common tubing fittings. As an alternative, a polymer stator face assembly can be placed onto a metal stator. However, this again adds volume and an additional sealing interface.
As a different type of example of the difficulty of making scientific instrument valves that have a long life, consider the design of the recently introduced EXL Technology valves made by Valco Instruments Co., Inc. This valve tries to achieve long lifetime by using some of the best materials, but which are still inadequate in life time when run at high pressure. Instead of trying to solve the lifetime problem with improved materials, the EXL Technology uses passive feedback, whereby the fluid pressure itself is used to help achieve the sealing force between the rotor and stator. The sealing force is limited to the active system pressure. For example, when the valve is operating at 5000 psi, a high force is used; when operating at only 2000 psi a lower force is used. However, the design of this valve necessitates that the fluid is connected by a tee to some non-switching stream volumes within the valve. These regions of the valve should not usually be exposed to the flowing stream, because of a concern for contamination and/or dead volume.
A rotary fluid valve which had a very long lifetime, which could be constructed at moderate cost, and which did not unduly increase the passage lengths, would be of value.
In accordance with one embodiment of the present invention, a rotary fluid switching valve is provided that has an exceptionally long life. The valve has a stator and rotor with sealing faces that press against each other with a high force as the stator pivots. One of the sealing faces is made of Tungsten Carbide/Carbon, referred to herein as WC/C, and the other is made of a fluorocarbon polymer. These two materials pressing against each other with a high pressure, as one slides on the other, were found to have an exceptionally long lifetime.
The WC/C is preferably a coating containing tungsten carbide particles in a soft amorphous carbon matrix. WC/C is preferably a plating on a metal stator body, such as stainless steel, with the body having machined threaded ports for connection to tube fittings. The stator is preferably formed of the fluorocarbon polymer. The polymer can be a solid fluorocarbon polymer, or a non-fluorocarbon polymer containing a fluorocarbon filler.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.